ISO week-numbering year incorrect for some years about angular HOT 10 CLOSED

Yes, I have a pending PR at #53879, we'll test it against Google's codebase to see how breaking the fix is consideredd !

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FYI added a link to a reproduction in StackBlitz - just the simple code that generates the table in the description screenshot.

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Quick investigation, this issue happens every year where Dec 31st is a sunday.
Looks like there is a bug in our algorithm.

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It looks like to boil down to: What do we consider the start of the week ?

Should it be sunday like in the US ? Or should it be monday like defined by ISO 8106.
Angular currently implements the former.

Repro : https://stackblitz.com/edit/angular-zcyztw?file=src%2Fmain.ts

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Also I wanted to add about this topic:

I believe the definition of the week numbering year / end of week is unclear in the framework : Is it ISO based or Locale based ?

In the case of date-fns, it has 2 functions getISOWeek() and getWeek(). The Former is ISO based while the latter is Locale based. luxon has the same distinctions : kkkk is the ISO week year, iiii is the locale week year.

In both cases Angular has a bug, the concept of week is neither locale based (the dedicated function doesn't rely on a locale or the dedicated function we have getLocaleFirstDayOfWeek) nor ISO based (is assumes the week starts on Sunday).

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Thanks @JeanMeche for looking into this.

Good catch on the "local" vs "ISO" week year - I didn't even notice local options in formatting and assumed week years was entirely from an ISO perspective.

• Using the "local week numbering year" in both date-fns and luxon matches what DatePipe is doing if I use the US locale (which is where I am) - which makes sense considering you're saying Angular is using the US-type definition of "start of week = Sunday".
• Using other locales like French (👍), though, matches the ISO week year as expected, since the start of the week is Monday.

Not that I have much say in how you all proceed, but I guess it seems like Angular is all about honoring the locale of the browser/system, so maybe defaulting to "local" week behavior for day of week, week number and week-numbering year makes the most sense?

I guess it would make sense to have formatting string options for both the "local" and "ISO" week items also?

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Maybe I spoke to soon - it looks like there is still a discrepancy in how date-fns and luxon handle local week years versus DatePipe.

Added the local week year's to the repro link, and right in 2020 you can see they differ from the DatePipe...but I was thinking since Angular was assuming Sunday as the start of week, they'd match. 🤔

Also discovered another online tool that works well for ad-hoc comparison of the "local" vs "ISO" week numbers and years: https://www.calendar-12.com/week_number. Basically let's you see the "north american" version where the week starts on Sunday, vs the "ISO"/everyone else version where the week starts on Monday.

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Supposing you're in the US, you locale is en-US.

For this locale, if at least 1 day of the week is the next year, the week is from next year (new Intl.Locale('en-US').weekInfo.minimalDays on Chrome browsers, or check the CLDR).

So for locale week year this is correct (and anguar is wrong by mixin ISO (thursday bases the weekyear) and week starting on Sunday).

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@JeanMeche Thanks again. So it sounds like you have all the info you need to adjust how Angular is calculating ISO weeks and week years?

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